FIG. 1 illustrates an isolated flyback Switch Mode Power Supply (“SMPS”) 100. The AC mains 101 supply the isolated flyback SMPS 100 and the AC mains 101 is connected via a bridge rectifier 102, which includes diodes D1 103, D2 104, D3 105 and D4 106, to an input capacitor C1 107.
SMPS control IC1 108 drives MOSFET switch S1 109. Through transformer T1 110, the energy is transferred to the output capacitor C2 111, using the Synchronous Rectifier (“SR”) MOSFET S2 112. MOSFET S2 112 is driven by the secondary side controller IC2 113. The output voltage 114 is controlled by secondary side controller IC2 113 using signal transformer T2 115.
The input side and the output side are electrically isolated by transformer T1 110 and transformer T2 115.
In a flyback converter, MOSFET S1 109 is switched on and energy is stored in transformer T1 110 during the primary stroke.
During the secondary stroke, MOSFET S2 112 is switched on and the energy is released to the secondary side.
FIG. 2 illustrates a timing diagram 200 showing the gate signals 201 of MOSFET S1 109 and gate signals 202 of MOSFET S2 112 with the drain1 signal 203, drain2 signal 205 and the current through MOSFET S2 112, Idrain2 204.
To achieve Zero Voltage Switching (“ZVS”) for the primary side switch MOSFET S1 109, the secondary side switch MOSFET S2 112 may be controlled by IC2 113.
Using IC2 113 to control MOSFET S2 112, FIG. 2 illustrates a timing diagram 200 for Discontinuous Conduction Mode (“DCM”) operation. During this operation, MOSFET S2 112 is switched on for a second time after the secondary stroke. During the conduction time (i.e. the Bi-directional Flyback Action (“bidifly action”) with the secondary current flowing in the opposite direction), energy is built up in transformer T1 110, which is released when MOSFET S2 112 is switched off. The energy in transformer T1 110 causes the drain node on the primary side, drain1 203, to drop to (almost) zero, after switch MOSFET S1 109 is switched on for the next cycle.
Using IC2 113 to control, FIG. 3 illustrates a timing diagram 300 for Quasi Resonant (“QR”) operation. During this operation, MOSFET S2 112 is kept in an on-state after the secondary stroke has ended. During the additional conduction time, energy is built up in transformer T1 110 which is released at the moment MOSFET S2 112 is switched off. The energy in transformer T1 110 causes the drain node on the primary side, drain1 301, to drop to (almost) zero, after which MOSFET S1 109 is switched on for the next cycle.
Switching on MOSFET S2 112 in the top of the drain1 301 signal is necessary because MOSFET S2 112 then switches on with ZVS. If MOSFET S2 112 must be switched on at the top of the drain1 203 signal, the moments to switch on MOSFET S2 112 for the second time may not be chosen freely as the tops of the drain1 203 signal appear at discrete time moments.
For ZVS, MOSFET S1 109 must be switched on after the bidifly action and at the moment the valley is reached for the drain1 301 signal.
If MOSFET S1 109 is switched on in the valley and MOSFET S2 112 is switched on at a top (or kept conductive after the secondary stroke), the time period, Tperiod 302, and thus the switching frequency (1/Tperiod) is dependent on the output power.
FIG. 4 illustrates a graph 400 of frequency versus output power curves. For a flyback SMPS, where the maximum frequency is limited and the Ipeak is variable for maintaining the QR operation, FIG. 4 illustrates the frequency versus output power curves.
FIG. 4 illustrates that it is not possible to maintain ZVS for MOSFET S1 109 and MOSFET S2 112 for any output power, while also having a fixed switching frequency.
Maintaining ZVS and having a fixed switching frequency (i.e. Frequency 1 405) is only possible for output power Power3 403, Power2 402 and Power1 401. However, for output power Power4 404, Frequency2 406 or Frequency3 407 is required.
Therefore, if a fixed frequency operation is needed, for example to limit the interference with a touch-screen controller of a portable device, this control method cannot be used.
In order to operate on a fixed switching frequency, flyback SMPS leave out the bidifly action and do not switch on MOSFET S1 109 in a valley.
FIG. 5 illustrates a timing diagram 500. In FIG. 5, MOSFET S1 109 is switched on while the drain1 501 voltage is not (close to) zero. Although fixed frequency operation is now achieved, switching losses increase.